A folded structure is a fabric made substantially by folding regularly at many places simultaneously at least one planar blank of a flat material being available as a thin flexible sheet or, a more rigid panel. This fabric has a periodic pattern of bilaterally arrayed elevations and depressions. This pattern is thereafter called “relief pattern”. The originally planar blank is subdivided into a plurality of plane segments, that is, into areal entities being small sections within the initially continuous material.
The folded structure extends within an X-Y-plane, thereafter called “zero plane”. Because of its relief it features a height in the Z-direction perpendicular to the zero plane—in a proportion between two times the edge length of a segment and two times the length of the relief pattern's basic unit in X-, or Y-direction.
Miura folded structures according to patent JP 3673519 B2, having a zigzag relief pattern, folded from their planar blank without holes and slits, have the disadvantage to be unfoldable back into the initial planar blank's zero plane. Thus, they can't neither transfer forces by themselves nor keep their shape under load. They are fixed only in conjunction with a plane skin layer. Hereby, they are each only a folding core of a sandwich panel. Besides, this folded structure is subject to geometrically induced constraints in detail at each meeting point of four folds, especially when the flat initial material has a notable thickness. Also oblong areal horizontal chamfers of the folds according to FIG. 2 of Koryo Miura's patent application publication DE 2213472 A, 1972, don't change this principally. This applies as well to somewhat larger areal chamfers of the folds according to FIG. 1 of patent US 2011/0281080 A1 being visible as valleys and ridges whose course is ascending and descending.
Despite of its hinged folds, the folded structure is driven apart again by the restraints in the folds' vertex points. That is why at numerous places each blank needs to be conducted not only to initialize the folding process but also during the folding process. In the case of a compactly outlined blank, this happens by bars at scissor grids. In the case of a continuous fabrication, this happens according to U.S. Pat. No. 6,913,570 B2 by a series of pairs of rolls whose jackets have a different relief or, by an increasing roll width according to U.S. Pat. No. 7,758,487 B2, or as well by an increasing relief depth of the rolls' interlocking surfaces. The intrinsic restraints of the completely folded Miura folded structure can be mitigated if the fold lines are made very deep and large. This again weakens it as a whole.
A lot of glue has to be used to enlarge the junction of such a folded structure to the skin layers. It has to be viscous and it must cling to the neighboring faces after calefaction. (products of the company Foldcore GmbH, Stuttgart, Germany, http://www.foldcore.eu/index.php?option=com_content&view=article&id=70:verbindungstechnik&catid=42:verbindungstechnik&Itemid=70). Besides, without reinforcement by impregnation or coating with resin, the long folds running from top to bottom and being still situated unattached in a supporting core's hollow space, that is, being not tied to the skin layers can deviate easily in parts from the provided fold line and, give way.
Folded structures having the well-known hexagonal honeycomb pattern for a sandwich panel's core layer, nowadays folded from one single flat but holey blank from a roll can be glued together already within themselves at many segments' surfaces indeed in order to be not unfoldable back (Pflug, J., Verpoest, I., Vandepitte, D.: Folded Honeycombs—Fast and continuous production of the core and a reliable core-skin bond, K. U. Leuven). However, when implemented later in a sandwich panel, not all of the segments standing upright in a completed honeycomb layer are bonded on their borders via additional, mostly small segments being glue tabs. Besides there are segments being doubled by being glued together extensively face to face, as well as segments remaining still separate. Hereby, the structure is oriented, that is, it is anisotropic.
The regular three-dimensional structure in patent WO 2009/087304 A1 disclosed as “Structure Tridimensionelle” not being foldable flat even without skin layers and being in some degree directionless constitutes the recent state of the art of a folded structure. It consists of at least two ply sheets each from a folded blank of a plane planar blank material whereby all fold lines are parallel. This folded structure is already based on a perfect dense packing, as a spatial pattern, of tetrahedra being chamfered slightly on two edges and, of slightly truncated pyramids. This geometry is known from plane space frame truss grids, albeit without chamfers.
But here again, both ply sheets substantially meet in detail only edge to face, that is, not face to face. That is why also here a lot of viscous resin is needed to achieve really the merely linear junctions to the segments of another ply sheet. That is why the folded fibrous material such as Aramid paper is rather soaked and coated with resin than only impregnated. Otherwise, the free Loose edges of the sloped small segments could simply buckle and give away. Or alternatively, a very rigid homogeneous planar blank material ought to be applied whose folds being bending-resistant and acting hereby like restrained in a clamp would cause the needed robustness. This could apply only to sheet metal without perforated fold lines. But his in turn would impede folding and, give to it the nature of pressing. Thus, the advantages of folding would be counteracted.
The large, evenly provided holes of the one only holey of both different ply sheets lie close together. Hereby, after cutting them, between these holes, only small links remain for the truncated pyramid tops. Because these links are small segments they can act only as equally small contact-faces for areal gluing.
Two ply sheets being folded according to this patent and glued together sufficiently tight are, as a folded structure, bending-resistant in one direction of the zero plane indeed but can be easily bent elastically In the other one because only the thickness of the blank's material contributes to the bending-resistance. Only by means of a third ply sheet a bending-resistant folded structure is then achieved by a static system with plate action in a small scale. With sufficiently resin glue this folded structure is so robust that it can serve by itself as a supporting structure. Only by this third folded ply sheet, the structure becomes geometrically equivalent in X- and Y-direction—as well as in Z-direction, that is, to the top and to the bottom. Hereby, it becomes equally stiff hereby, that is, as directionless as possible, anisotropic whereby a certain inexactness remains in the detail of the differently high layered horizontal faces. This inexactness has to be mastered while adding plane skin layers with a lot of resin glue or, by the yieldingness of the not yet soaked material.
Hereby, the seeming advantage of folded structures—to manage as a lightweight product with less input of material and energy not only not before usage but also already during its production—cannot be de facto completely exploited. That is why for the same intended use with an equally high technical effort, further on equally robust alternatives are available, These are panels being produced otherwise than by folding. They have to be mentioned because some aspects of their relief patterns are relevant for the examples of folded structures according to this invention.
Alternatives of application to the variant of the “Structure Tridimensionelle” with only two folded ply sheets could be patterned one-ply core layers each being deep-drawn and then welded to plane skin layers of a structured hollow core panel of thermoplastic material. on the one hand a core layer having a relief pattern of alternatingly opposed small waves, or mounts, bilaterally oriented to the top and to the bottom (U.S. Pat. No. 5,400,918); on the other hand a core layer whose frusto-conical bumps are arrayed in the same symmetry of pattern (of the firm Triplex GmbH, Haiterbach, Germany).
Alternatives of application to the variant of the “Structure Tridimensionelle” with one additional ply sheet, that is, with three ply sheets then could be structures of two superimposed ply sheets each having a relief pattern of truncated pyramids. On the one hand, this could be this could be pressing natural fibers in a matrix of resin needing time for drying, as a relief pattern of larger acute quadrilateral pyramids having rounded edges, arrayed without distance (Wonneberger, M.—Invent GmbH—and Leutermann, H.—Pyra Tec GmbH—: “Innovative Wabensysteme für Leichtbauanwendungen”, 2005), on the other hand, pressing (sheet metal), injection molding, deep-drawing, (plastics), or laminating (synthetic resin) of weak fibrous materials, as a relief pattern of spaced-apart 3-, 4-, 6-, or 8-sided truncated pyramids (DE 199 44 662 C5: “Räumliche Tragkonstruktion aus flächigen Formbauteilen”, 2009, firm Borit, Aachen).